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"If the temperature is more than 400 degrees C (750 degrees F), what should be the basic composition of the grease or lubricant?"
Many factors affect the selection of a lubricant’s composition. With respect to temperature, the major contributors are viscosity, thermal degradation and oxidation. In a perfect world, if you focused solely on temperature to drive lubricant selection, you would want a lubricant that was pumpable at low temperatures while still providing the protection at high temperatures (very high viscosity index), that would not thermally degrade or “cook” onto hot machine surfaces and leave deposits, and that would not oxidize at the elevated temperatures. This means that a hydrocarbon-based lubricant would not be an option.
Even polyalphaolefin (PAO) synthetics, which are the purest and best-performing of the hydrocarbon base oils, cannot withstand the temperatures mentioned here. The practical temperature limits for synthetic hydrocarbons are less than 200 degrees C. Their initial thermal degradation temperatures are higher than this, but they oxidize very rapidly at these elevated temperatures, effectively making their lifespan only a few hours.
Effective lubrication is only available at temperatures below 170 degrees C. Anything above this temperature will almost always require a compromise to be made. These compromises come in the form of reduced life, lower load-carrying capacity, slower speed, higher levels of friction, difficulties in application, compatibility issues, etc.
The thermal decomposition temperature for base oils can be tested using ASTM D2879. These test results cannot be improved upon with the use of additives, but they can be reduced by the presence of less stable compounds found in the lubricant including additives. The following are thermal decomposition temperatures for some popular base oils:
As you can see, the choices become very limited above 400 degrees C. There are lubricants that can be used well above this temperature range, but the problem with them is that they are solids at normal temperatures, which poses a huge problem for transferability.
When discussing the upper temperature limits of a grease, you also must include any limitations of the thickener. When the temperature of the grease is raised, a point is reached where the gel structure is destabilized and the grease becomes a liquid. This temperature is called the dropping point. Some of the most stable, conventional grease thickener dropping points are listed below:
Obviously, conventional grease thickeners also cannot come close to withstanding 400 degrees C. A probable solution is to look toward solid lubricants and composites. While there are many disadvantages to using a solid lubricant, sometimes compromises must be made to achieve better performance of certain properties.
As far as lubricant selection, there are many choices for temperatures below 100 degrees C, which makes the decision more difficult. However, for temperatures above 400 degrees C, only a small handful of lubricants are available, making the decision process much easier. The only lubricants with long life at these temperatures are liquid metals, liquid oxides, glasses and a few solid lubricants like molybdenum disulfide.
The only real solution for these extremely high-temperature lubrication problems is to re-evaluate the system and environment to design a situation where the lubricant can have a fighting chance.